High Temp Limit 1

[brentie]

I am currently searching for a limit switch that can handle 420 dg/F. We have a oven and trays slide in, the limits are mounted at the exit do tell us if all trays are inside. They are currently outside of the oven since the trays protrude in the front in back.

ex ____------___LIMIT The solid line is the tray the


dotted is the oven. The word limit is where the limit is mounted. Any ideas?

 

[Skogsgurra]

brentie,

I have used coils embedded in a ceramic clay (talk to a potter near you). The coils have a rather low inductance when there is no iron close to them. Iron increases inductance and that can be sensed in several ways. The coils are in effect the same thing as you use in a metal detector so they will akso work with non-ferrous metals. But then it is not the inductance that changes. Instead, the losses go up when metal comes close (Q factor goes down).

If the oven can accept an extra air flow, then pressurized air can easily sense a moving vane that is actuated by the trays. A fork configuration (air nozzle in one leg and sensing nozzle in the other leg, vane goes in between the legs) works very well and is very reliable. I used 100 PSI and solid state pressure sensors (monted separately outside the oven, of course ;-) ) with great sucess in very harsh environment. The nozzles tend to clog in some carbon rich (oil burners) environments so it is best place the sensors on a bracket that can be drawn out of the oven for cleaning.

You have probably already thought of the obvious solution: let the trays actuate an arm that turns a shaft and monitor the turning outside the oven. Some ovens do not like having holes in them so it is not always possible to use this simple technique.

Good luck!

 

[hacksaw]

light curtains are also used

 

[cbarn24050]

try a glass reed switch with a magnet.

 

[nbucska]

cbarn:
use magnet with high enough Curie point !!

<nbucska@pcperipherals.com>